The statements in this section merely provide background information on embodiments of the present disclosure and do not constitute prior art.
A video image is formed of a series of pictures, and each picture is divided into predetermined regions such as blocks. When the region of an image is divided into blocks, the divided blocks are classified into an intra block and an inter block depending on an encoding scheme. The intra-block refers to a block that is encoded based on an intra-prediction coding scheme. The intra-prediction coding scheme predicts pixels of a current block by using pixels of neighboring blocks that have been already encoded and decoded within a current picture, so as to generate a predicted block, and encodes pixel differences between the predicted block and the current block. The inter-block means a block that is encoded based on an inter-prediction coding scheme. The inter-prediction encoding scheme predicts a current block in a current picture by referring to at least one previous picture and/or at least one subsequent picture, so as to generate a predicted block, and encodes the difference between the predicted block and the current block. Here, a picture that is referred to in encoding or decoding the current picture (i.e., current frame) is called a reference picture.
The inventor(s) has noted that the general purpose of image signal coding is to reduce data volume of the image, that is, to perform an image compression. The inventor(s) has noted that the image compression takes the prediction in two ways, that is, an intra prediction and an inter prediction. The prediction serves to generate predicted image signals similar to original signals and transmit differences between the original signals and the predicted signals. Assuming pixel values are presented with 100, 200, 150, 240 and 178, their binary numbers are expressed as 100=1100100(2), 200=11001000(2), 150=10010110(2), 240=11110000(2) and 178=10110010(2), 40 bits in total. However, when these values are predicted into, e.g. 95, 190, 149, 240 and 178 and encoding is performed on residual signals that are the differences between the predicted values and the original values, the encoded result is expressed by 5=101(2), 10=1010(2), 1=1(2) and 0=0(2) which mean substantially reduced bits used in encoding and in turn reduced size of the overall image to be transmitted to a decoding apparatus. In other words, the inventor(s) has noted that the better prediction achieves the higher coding efficiency.